Plumbing Engineer August 2021/59
Industry News
59/Plumbing Engineer August 2021
D
uring 2020, Peter Skinner, P.E. and Gary Klein wrote a series of seven articles for Plumbing Engineer providing an in-depth look into the engineering decisions made in the design and construc- tion of three 35-unit apartment buildings in upstate New York. Those articles focused on the effort to right-size the plumbing and the water heaters for customer satisfaction and energy efficiency. In this article, we detail how right-sizing also directly impacts Legionella growth in building water systems. When Peter and Gary began the engineering review of the premise plumbing and water heating design, the foun- dations for the three buildings in Phase II were already completed. This meant all the big architectural decisions had already been made and were literally and figuratively cast in concrete. As with many things related to dwellings, it is all about location, location, location. Three decisions that should have been considered in the schematic design of the build- ing are: 1. Location of the wet rooms in each apartment relative to each other and to the source of hot water within the apartment. 2. Location of the wet rooms in each apartment relative to those in the adjacent apartment. 3. Location of the mechanical room relative to all apart- ments. Time-to-Tap The first principle is determining time to tap - how long the tenants wait for hot water. This issue applies to a 35-unit apartment as much as a 350-bed hospital or a 35-story office building. This decision rests with the architect. The plumbing engineer and the plumber cannot overcome the impact of an inherently inefficient architectural building layout design. In the simplest of terms, the closer the fixtures are to each other and the source of hot water, the less time it takes and the less water and energy that runs down the drain. This is the reason why ASHRAE Guideline 12 states: "Planning to control the conditions that increase the poten- tial for Legionella growth should begin with the architec- ture, design, and engineering of buildings. Building layout and location of water fixtures will significantly affect the number, length, and complexity of pipe runs; the occur- rence of dead legs; and the use of water and energy." ASPE says that a good time-to-tap is no longer than 10 seconds. Research conducted by Carl Hiller almost 20 years ago documented that the source of hot water cannot be more than 5 seconds away from each hot water fixture to deliver 105 F or hotter within 10 seconds. This is very close. How close? It depends on the volume (length and diameter) in the pipe between the source of hot water and each fixture. The pipe length has already been determined by the architect. Most plumbing codes stipulate a minimum nominal diameter of 0.5 inches on branches and twigs. Some codes allow as small as 0.375 inches for some fixtures. Using the IAPMO Water Demand Calculator enables the right-sizing of the trunks and branches. Additional information, which is available, is needed on the pressure drop in 0.375-inch pipe and fittings to enable a plumbing engineer to be able to make a case for its use on the twigs. Why is time to tap so important? Customer satisfaction increased; Water and energy loss reduced while waiting for hot water to arrive; Energy loss further reduced when the piping in the By Tim Keane and Gary Klein
Legionella: Cost-Effective Control, Part 2
Mitigating Legionella risk by design.
Photo: iStock.com/jarun011
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